To better understand the content (e.g., hydrocarbons, fresh water, etc.) of a reservoir in a subterranean formation, characterization of the reservoir can be performed using geological modeling techniques. Modern geological modeling techniques are leading the industry to routinely build relatively large and detailed three-dimensional geocellular models. These models currently may range in size from 10 to 100 million grid cells and are growing. This has resulted in a steadily increasing gap between flow simulation capability and the desire to build geologic-scale reservoir simulation models.
In addition to sheer size, strong heterogeneity in the geological models may create computational problems during flow simulation. Geological models may need to use relatively small cells that have highly contrasting reservoir properties in order to adequately capture reservoir heterogeneity. Moreover, within a geological model, there may often be a relatively small proportion of active (porous and permeable) cells that are sparsely distributed. These factors may result in relatively complex hydraulic connectivity. Traditional finite difference flow simulators are not designed to handle such geological models efficiently and often have to resort to a processing of upscaling the grid to achieve practical computational times. This upscaling process however reduces the resolution to which flow behavior can be accurately resolved thus losing the benefits of the detailed geological characterization.
Additionally, the presence of fractures in a reservoir can pose an additional challenge when modeling the reservoir. The common approach to modeling fractured reservoirs has been to idealize the fractured reservoir as a dual porous and permeable medium by interacting matrix and fracture grid cells. This concept has been extended to multiple interacting porous mediums for very complex fractured, typically carbonate, reservoirs. Typically, this is an acceptable method for reservoirs that are dominated by small scale fractures, typically much smaller that simulation grid cells. Comparatively there are typically few fracture corridors present in reservoirs and these large structures can act as flow barriers if they are cemented or as fluid highways. Fracture corridors are an extraordinary cluster of a large number of quasi-parallel fractures. They can be deterministically described with the help of reservoir characterization techniques and they present one of the major factors affecting the flow in the reservoir. Although the dimensions of fracture corridors are much larger than the dimension of a single fracture, there is still a problem with resolving fracture corridors using the standard coarse scale simulation grid: the thickness of fracture corridors is much smaller than the typical size of the cell and also the direction could be different from the grid orientation. Without proper resolution there is no confidence in obtained results.